1. Field of the Invention
The present invention relates to a pressure-sensitive sensor for detecting an external load using a pressure-sensitive conductive rubber, and a grip apparatus and a robot manipulator equipped with the same.
2. Description of the Related Art
Conventionally, there is known a pressure-sensitive sensor including a pressure-sensitive conductive rubber made by containing conductive particles such as fine metal particles in rubber. When no load is applied, since conductive particles contained in the rubber have no or little contact with each other, the pressure-sensitive conductive rubber is electrically insulated or in a highly resistive state. Then, when load is applied to press on the pressure-sensitive conductive rubber, the rubber part is deformed to bring the conductive particles come close to each other in the deformed portion. In this case, since the number of conductive particles in a contact state increases, the electric resistance value of the pressure-sensitive conductive rubber decreases as a whole. The pressure-sensitive sensor takes advantage of the characteristic of changing the electric resistance value according to the load on the pressure-sensitive conductive rubber.
As this kind of pressure-sensitive sensor, there is known a pressure-sensitive sensor disclosed in Japanese Patent Application Laid-Open No. H09-17276. The pressure-sensitive sensor in Japanese Patent Application Laid-Open No. H09-17276 has a flat plate of pressure-sensitive conductive rubber arranged on electrodes partially provided on a substrate, and a flexible sheet member made of thin layers of resin. The pressure-sensitive sensor also has a spacer for supporting, with a given gap, a part formed by putting the flexible sheet member on the pressure-sensitive conductive rubber. This spacer is provided on the surface of the substrate. Further, the pressure-sensitive sensor has a buffer material made of rubber or the like and provided on a surface opposite to the surface of the flexible sheet member supported by the spacer, and a cover member, which integrally covers the spacer, the flexible sheet member, and the buffer material, and is bound onto the surface of the substrate. In Japanese Patent Application Laid-Open No. H09-17276, load applied to the cover member is buffered and dispersed by the buffer material and the flexible sheet member so that the load will be gradually transmitted to the pressure-sensitive conductive rubber. Thus, since the load pressing on the pressure-sensitive conductive rubber per unit area is reduced, the electric resistance value does not change sharply, enabling widening of a load detection range.
Further, since the pressure-sensitive conductive rubber is made by containing conductive particles in rubber, there are the hysteresis characteristics of rubber whose deformation due to the application or release of load remains, and the creep characteristics of rubber continuing to deform slowly with a certain constant load. Because of the hysteresis characteristics and creep characteristics of rubber, the change of electric resistance value with respect to the load becomes nonlinear. Japanese Patent Application Laid-Open No. 2004-340731 discloses a pressure-sensitive sensor for reducing this nonlinearity. The pressure-sensitive sensor disclosed in Japanese Patent Application Laid-Open No. 2004-340731 includes a pressure-sensitive conductive rubber formed in the shape of a disk having flat surfaces on top and bottom, electrodes arranged on the top and bottom surfaces of the pressure-sensitive conductive rubber to face each other, and an enclosure member made of metal or resin to constrain the deformation of the pressure-sensitive conductive rubber in a direction different from a direction in which load is received around the pressure-sensitive conductive rubber. Since the deformation of the pressure-sensitive conductive rubber in a radial direction is constrained to make the pressure-sensitive conductive rubber deform only in a thickness direction as the load application direction, an increase in the contact areas between the pressure-sensitive conductive rubber and the electrodes are restricted upon application of load to reduce the nonlinearity between the load and the electric resistance value.
However, the structure having a gap between a pressure-sensitive sensor and a member for transmitting load to the pressure-sensitive sensor, like the gap between the pressure-sensitive conductive rubber and the flexible sheet member in Japanese Patent Application Laid-Open No. H09-17276, cannot obtain the sensor's output for a minute amount of load (e.g., load no more than 1N). The reason why the load cannot be detected by the pressure-sensitive conductive rubber is because the load has to continue to be applied until the gap between the pressure-sensitive conductive rubber and the member for transmitting the load to the pressure-sensitive conductive rubber disappears in order to apply load to the pressure-sensitive sensor and transmit the load to the pressure-sensitive conductive rubber. In other words, in order to detect a minute amount of load, it is desired to provide such a structure as to eliminate a gap between the pressure-sensitive conductive rubber and the member for transmitting load to the pressure-sensitive conductive rubber or to make the gap vanishingly small. Therefore, it is better to provide such a structure as to apply a pressing force (preload) between the pressure-sensitive conductive rubber and the member for transmitting load to the pressure-sensitive conductive rubber so that even a minute amount of load will be transmitted to the pressure-sensitive conductive rubber.
However, since the pressure-sensitive conductive rubber is made by containing conductive particles in rubber, if preload is imposed between the pressure-sensitive conductive rubber and the member for transmitting load to the pressure-sensitive conductive rubber, the electric resistance value of the pressure-sensitive conductive rubber will be reduced with time due to the creep characteristics of rubber as shown in FIG. 6. Although the nonlinearity between the load and the electric resistance value due to the hysteresis characteristics and creep characteristics of rubber is reduced in Japanese Patent Application Laid-Open No. 2004-340731, the creep characteristics is not improved in nature. Thus, in Japanese Patent Application Laid-Open No. 2004-340731, since the electric resistance value is reduced with time in such a state that preload is imposed on the pressure-sensitive conductive rubber, an increase in detection accuracy of load on the pressure-sensitive sensor leads to false detection of load. In this case, the detection accuracy of load cannot be increased and hence a minute amount of load cannot be detected.
Therefore, it is an object of the present invention to provide a pressure-sensitive sensor capable of increasing the detection accuracy of load on a pressure-sensitive sensor to detect a minute amount of load even when preload is imposed on a pressure-sensitive conductive rubber to reduce the electric resistance value of a pressure-sensitive conductive rubber with time due to the creep characteristics of rubber.